Planetary gear train of automatic transmission for vehicles

ABSTRACT

A planetary gear train of an automatic transmission may include a first planetary gear set including a first sun gear, a first planet carrier, and a first ring gear, a second planetary gear set including a second sun gear receiving an inverse rotation speed from the first ring gear, a second planet carrier, and a second ring gear selectively connected to the first planet carrier or the first ring gear, a third planetary gear set including a third sun gear selectively connected to the first planet carrier, the first ring gear, or the second ring gear, a third planet carrier, a third ring gear selectively connected to the first sun gear, three transfer gears engaging three rotation elements of the first planetary gear set with selected rotation elements, and five frictional elements selectively interconnecting the rotation elements or selectively connecting the rotation elements to a transmission housing.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2012-0119958 filed on Oct. 26, 2012, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic transmission for avehicle. More particularly, the present invention relates to a planetarygear train of an automatic transmission for a vehicle that can improvemountability by reducing a length thereof and reduce fuel consumption byimproving power delivery performance.

2. Description of Related Art

Typically, a planetary gear train is realized by combining a pluralityof planetary gear sets and friction members. It is well known that whena planetary gear train realizes a greater number of shift speeds, speedratios of the planetary gear train can be more optimally designed, andtherefore a vehicle can have economical fuel mileage and betterperformance. For that reason, the planetary gear train that is able torealize more shift speeds is under continuous investigation.

Though achieving the same number of speeds, the planetary gear train hasa different operating mechanism according to a connection betweenrotation elements (i.e., sun gear, planet carrier, and ring gear). Inaddition, the planetary gear train has different features such adurability, power delivery efficiency, and size depend on the layoutthereof. Therefore, designs for a combining structure of a gear trainare also under continuous investigation.

In addition, the planetary gear train realizes a plurality ofshift-speeds. However, another friction member must be operated afterone friction member is released so as to shift to a neighboringshift-speed from a view of shift control. In addition, a step ratiobetween the neighboring shift-speeds should be controlled to be suitableaccording to the planetary gear train.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing aplanetary gear train of an automatic transmission for a vehicle havingadvantages of improving mountability by shortening a length thereof andreducing fuel consumption by improving power delivery performance as aconsequence of achieving eight forward speeds and one reverse speedhaving excellent operating condition of frictional elements and stepratios by combining three planetary gear sets separately disposed on afirst shaft and a second shaft, three externally-meshing gears, and fivefrictional elements.

In an aspect of the present invention, a planetary gear train of anautomatic transmission for a vehicle may include a first shaft receivingtorque of an engine, a second shaft disposed in parallel with the firstshaft, a first planetary gear set disposed on the first shaft, andincluding a rotation elements thereof a first sun gear selectivelyoperated as an output element or a fixed element, a first planet carrierdirectly connected to the first shaft and continuously operated as aninput element, and a first ring gear operated as an output element, asecond planetary gear set disposed on the second shalt, and including asrotation elements thereof a second sun gear receiving an inverserotation speed from the first ring gear, a second planet carrierdirectly connected to the second shaft, and a second ring gearselectively connected to the first planet carrier or the first ringgear, a third planetary gear set disposed on the second shaft, andincluding as rotation elements thereof a third sun gear selectivelyconnected to the first planet carrier, the first ring gear, or thesecond ring gear, a third planet carrier directly connected to thesecond shaft and an output gear, a third ring gear selectively connectedto the first sun gear, three transfer gears engaging three rotationelements of the first planetary gear set with selected rotation elementsamong the rotation elements of the second and third planetary gear sets,and five frictional elements selectively interconnecting the rotationelements of the first, second, and third planetary gear sets orselectively connecting the rotation elements to a transmission housing.

Each of the first, second, and third planetary gear sets is a singlepinion planetary gear set.

The three transfer gears may include a first transfer gear including afirst transfer drive gear connected to the first ring gear and a firsttransfer driven gear connected to the second sun gear, a second transfergear including a second transfer drive gear selectively connected to thefirst planet carrier or the first ring gear and a second transfer drivengear connected to the third sun gear, and a third transfer gearincluding a third transfer drive gear selectively connected to the firstsun gear and a third transfer driven gear connected to the third ringgear.

The second transfer driven gear is selectively connected to the secondring gear.

The five frictional elements may include a first brake mounted betweenthe first sun gear and the transmission housing, a first clutch mountedbetween the first sun gear and the third transfer drive gear, a secondclutch mounted between the first planet carrier and the second transferdrive gear, a third clutch mounted between the second ring gear and thesecond transfer driven gear, and a fourth clutch mounted between thefirst ring gear and the second transfer drive gear.

The third clutch is mounted between the second ring gear and the thirdsun gear.

The fourth clutch is mounted between the first transfer drive gear andthe second transfer drive gear.

A first forward speed is achieved by operating the first brake and thefirst and second clutches, a second forward speed is achieved byoperating the first brake and the first and fourth clutches, a thirdforward speed is achieved by operating the first, second, and fourthclutches, a fourth forward speed is achieved by operating the first,third, and fourth clutches, a fifth forward speed is achieved byoperating the first, second, and third clutches, a sixth forward speedis achieved by operating the second, third, and fourth clutches, aseventh forward speed is achieved by operating the first brake and thesecond and third clutches, an eighth forward speed is achieved byoperating the first brake and the third and fourth clutches, and areverse speed is achieved by operating the first brake and the first andthird clutches.

In another aspect of the present invention, a planetary gear train of anautomatic transmission for a vehicle, may include a first shaftreceiving torque of an engine, a second shaft mounted in parallel withthe first shaft, a first planetary gear set mounted on the first shaft,and including a rotation elements thereof a first sun gear selectivelyoperated as an output element or a fixed element, a first planet carrierdirectly connected to the first shaft and continuously operated as aninput element, and a first ring gear operated as an output element, asecond planetary gear set mounted on the second shaft, and including asrotation elements thereof a second sun gear receiving an inverserotation speed from the first ring gear, a second planet carrierdirectly connected to the second shaft, and a second ring gearselectively connected to the first planet carrier or the first ringgear, a third planetary gear set mounted on the second shaft, andincluding as rotation elements thereof a third sun gear selectivelyconnected to the first planet carrier, the first ring gear, or thesecond ring gear, a third planet carrier directly connected to thesecond shaft and an output gear, a third ring gear selectively connectedto the first sun gear, a first transfer gear including a first transferdrive gear connected to the first ring gear and a first transfer drivengear connected to the second sun gear, a second transfer gear includinga second transfer drive gear selectively connected to the first planetcarrier or the first ring gear and a second transfer driven gearconnected to the third sun gear, a third transfer gear including a thirdtransfer drive gear selectively connected to the first sun gear and athird transfer driven gear connected to the third ring gear, a firstbrake mounted between the first sun gear and the transmission housing, afirst clutch mounted between the first sun gear and the third transferdrive gear, a second clutch mounted between the first planet carrier andthe second transfer drive gear, a third clutch mounted between thesecond ring gear and the second transfer driven gear, and a fourthclutch mounted between the first ring gear and the second transfer drivegear.

Each of the first, second, and third planetary gear sets is a singlepinion planetary gear set.

The third clutch is mounted between the second ring gear and the thirdsun gear.

A first forward speed is achieved by operating the first brake and thefirst and second clutches, a second forward speed is achieved byoperating the first brake and the first and fourth clutches, a thirdforward speed is achieved by operating the first, second, and fourthclutches, a fourth forward speed is achieved by operating the first,third, and fourth clutches, a fifth forward speed is achieved byoperating the first, second, and third clutches, a sixth forward speedis achieved by operating the second, third, and fourth clutches, aseventh forward speed is achieved by operating the first brake and thesecond and third clutches, an eighth forward speed is achieved byoperating the first brake and the third and fourth clutches, and areverse speed is achieved by operating the first brake and the first andthird clutches.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary embodiment of the present invention.

FIG. 2 is an operational chart of friction members at each shift-speedapplied to a planetary gear train according to an exemplary embodimentof the present invention.

FIG. 3A is a lever diagram of a planetary gear train at the firstforward speed according to an exemplary embodiment of the presentinvention.

FIG. 3B is a lever diagram of a planetary gear train at the secondforward speed according to an exemplary embodiment of the presentinvention.

FIG. 3C is a lever diagram of a planetary gear train at the thirdforward speed according to an exemplary embodiment of the presentinvention.

FIG. 3D is a lever diagram of a planetary gear train at the fourthforward speed according to an exemplary embodiment of the presentinvention.

FIG. 3E is a lever diagram of a planetary gear train at the fifthforward speed according to an exemplary embodiment of the presentinvention.

FIG. 3F is a lever diagram of a planetary gear train at the sixthforward speed according to an exemplary embodiment of the presentinvention.

FIG. 3G is a lever diagram of a planetary gear train at the seventhforward speed according to an exemplary embodiment of the presentinvention.

FIG. 3H is a lever diagram of a planetary gear train at the eighthforward speed according to an exemplary embodiment of the presentinvention.

FIG. 3I is a lever diagram of a planetary gear train at a reverse speedaccording to an exemplary embodiment of the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

An exemplary embodiment of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

Description of components that are not necessary for explaining thepresent exemplary embodiment will be omitted, and the same constituentelements are denoted by the same reference numerals in thisspecification.

In the detailed description, ordinal numbers are used for distinguishingconstituent elements having the same terms, and have no specificmeanings.

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary embodiment of the present invention.

Referring to FIG. 1, a planetary gear train according to an exemplaryembodiment of the present invention includes first, second, and thirdplanetary gear sets PG1, PG2, and PG3, five frictional elements B1, C1,C2, C3, and C4, and three transfer gears TF1, TF2, and TF3.

The first planetary gear set PG1 is disposed on a first shaft IS1, andthe second and third planetary gear sets PG2 and PG3 are disposed on asecond shaft IS2 disposed apart from and in parallel with the firstshaft IS1.

Therefore, torque input from the first shaft IS1 is transmitted to thesecond and third planetary gear sets PG2 and PG3 through the firstplanetary gear set PG1, is converted into eight forward speeds and onereverse speed by operations of the first, second, and third planetarygear sets PG1, PG2, and PG3, and is then output through an output gearOG.

The first planetary gear set PG1 is a single pinion planetary gear set,and having a first sun gear S1, a first ring gear R1, and a first planetcarrier PC1 rotatably supporting a first pinion P1 engaged with thefirst sun gear S1 and the first ring gear R1 as rotation elementsthereof.

The second planetary gear set PG2 is a single pinion planetary gear set,and having a second sun gear S2, a second ring gear R2, and a secondplanet carrier PC2 rotatably supporting a second pinion P2 engaged withthe second sun gear S2 and the second ring gear R2 as rotation elementsthereof.

The third planetary gear set PG3 is a single pinion planetary gear set,and having a third sun gear S3, a third ring gear R3, and a third planetcarrier PC3 rotatably supporting a third pinion P3 engaged with thethird sun gear S3 and the third ring gear R3 as rotation elementsthereof.

The first planet carrier PC1 is directly connected to the first shaftIS1 so as to be always operated as an input element, the second planetcarrier PC2 and the third planet carrier PC3 are connected through thesecond shaft IS2, and the third planet carrier PC3 is directly connectedto an output gear OG so as to be always operated as an output element.

The first, second, and third transfer gears TF1, TF2, and TF3 may behelical gears and respectively have first, second, and third transferdrive gears TF1 a, TF2 a, and TF3 a and first, second, and thirdtransfer driven gears TF1 b, TF2 b, and TF3 b externally meshed witheach other.

The first transfer gear TF1 includes the first transfer drive gear TF1 adirectly connected to the first ring gear R1 and the first transferdriven gear TF1 b directly connected to the second sun gear S2.

The second transfer gear TF2 includes the second transfer drive gear TF2a selectively connected to the first planet carrier PC1 or the firstring gear R1 and the second transfer driven gear TF2 b directlyconnected to the third sun gear S3.

The third transfer gear TF3 includes the third transfer drive gear TF3 aselectively connected to the first sun gear S1 and the third transferdriven gear TF3 b directly connected to the third ring gear R3.

The first shaft IS1 or rotation elements of the first planetary gear setPG1 and rotation elements of the second planetary gear set PG2 or thethird planetary gear set PG3 connected by the first, second, and thirdtransfer gears TF1, TF2, and TF3 are rotated in opposite directions toeach other. Gear ratios of the first, second, and third transfer gearsTF1, TF2, and TF3 are set according to speed ratios demanded atshift-speeds.

Arrangements of the frictional elements B1, C1, C2, C3, and C4 will bedescribed in detail.

The first brake B1 is disposed between the first sun gear S1 and thetransmission housing H.

The first clutch C1 is disposed between the first sun gear S1 and thethird transfer drive gear TF3 a.

The second clutch C2 is disposed between the second planet carrier PC2and the second transfer drive gear TF2 a.

The third clutch C3 is disposed between the second ring gear R2 and thesecond transfer driven gear TF2 b.

The fourth clutch C4 is disposed between the first ring gear R1 and thesecond transfer drive gear TF2 a.

The frictional elements including the first, second, third, and fourthclutches C1, C2, C3, and C4 and the first brake B1 are conventionalmulti-plate friction elements of wet type that are operated by hydraulicpressure.

FIG. 2 is an operational chart of friction members at each shift-speedapplied to a planetary gear train according to an exemplary embodimentof the present invention.

As shown in FIG. 2, three frictional elements are operated at eachshift-speed in the planetary gear train according to an exemplaryembodiment of the present invention.

A first forward speed 1ST is achieved by operating the first brake S1and the first and second clutches C1 and C2.

A second forward speed 2ND is achieved by operating the first brake B1and the first and fourth clutches C1 and C4.

A third forward speed 3RD is achieved by operating the first, second,and fourth clutches C1, C2, and C4.

A fourth forward speed 4TH is achieved by operating the first, third,and fourth clutches C1, C3, and C4.

A fifth forward speed 5TH is achieved by operating the first, second,and third clutches C1, C2, and C3.

A sixth forward speed 6TH is achieved by operating the second, third,and fourth clutches C2, C3, and C4.

A seventh forward speed 7TH is achieved by operating the first brake B1and the second and third clutches C2 and C3.

An eighth forward speed 8TH is achieved by operating the first brake B1and the third and fourth clutches C3 and C4.

A reverse speed REV is achieved by operating the first brake B1 and thefirst and third clutches C1 and C3.

FIG. 3A to FIG. 3I are lever diagrams of the planetary gear train ateach shift-speed according to the first exemplary embodiment of thepresent invention, and illustrate shift processes of the planetary geartrain according to the first exemplary embodiment of the presentinvention by lever analysis method.

Referring to FIG. 3A to FIG. 3I, three vertical lines of the firstplanetary gear set PG1 are set as the first sun gear S1, the firstplanet carrier PC1, and the first ring gear R1 from the left to theright, three vertical lines of the second planetary gear set PG2 are setas the second sun gear S2, the second planet carrier PC2, and the secondring gear R2 from the left to the right, and three vertical lines of thethird planetary gear set PG3 are set as the third sun gear S3, the thirdplanet carrier PC3, and the third ring gear R3 from the left to theright.

In addition, a middle horizontal line represents a rotation speed of“0”, an upper horizontal line represents a rotation speed of “1.0”, anda lower horizontal line represents a rotation speed of “−1.0”.

“−” means that rotational elements is rotated in an opposite directionof a rotational direction of the engine. It is because the first shaftIS1 and the first planetary gear set PG1 are externally meshed with thesecond planetary gear set PG2 and the third planetary gear set PG3through the first, second, and third transfer gears TF1, TF2, and TF3without an idling gear.

In addition, the rotation speed of “1.0” represents the same rotationalspeed as the first shaft IS1 which is an input shaft. Distances betweenthe vertical lines of the first, second, and third planetary gear setsPG1, PG2, and PG3 are set according to each gear ratio (teeth number ofa sun gear/teeth number of a ring gear).

Hereinafter, referring to FIG. 2 and FIG. 3A to FIG. 3I, the shiftprocesses of the planetary gear train according to an exemplaryembodiment of the present invention will be described in detail.

[First Forward Speed]

Referring to FIG. 2, the first brake B1 and the first and secondclutches C1 and C2 are operated at the first forward speed 1ST.

As shown in FIG. 3A, the rotation speed of the first shaft IS1 is inputto the first planet carrier PC1, and the first sun gear S1 and the thirdring gear R3 are operated as fixed elements by operation of the firstbrake B1 and the first clutch C1.

The rotation speed of the first ring gear R1 is increased according tothe gear ratio of the first transfer gear TF1 and is then input to thesecond sun gear S2 as an inverse rotation speed, and the rotation speedof the first planet carrier PC1 is reduced according to the gear ratioof the second transfer gear TF2 and is then input to the third sun gearS3 as an inverse rotation speed by operation of the second clutch C2.

Therefore, the rotation elements of the third planetary gear set PG3form a first shift line SP1 such that D1 is output through the thirdplanet carrier PC3.

[Second Forward Speed]

The second clutch C2 that was operated at the first forward speed 1ST isreleased and the fourth clutch C4 is operated at the second forwardspeed 2ND.

As shown in FIG. 3B, the rotation speed of the first shaft 151 is inputto the first planet carrier PC1, and the first sun gear S1 and the thirdring gear R3 are operated as fixed elements by operation of the firstbrake B1 and the first clutch C1.

The rotation speed of the first ring gear R1 is increased according tothe gear ratio of the first transfer gear TF1 and is then input to thesecond sun gear S2 as an inverse rotation speed, and the rotation speedof the first ring gear R1 is reduced according to the gear ratio of thesecond transfer gear TF2 and is then input to the third sun gear S3 asan inverse rotation speed by operation of the fourth clutch C4.

Therefore, the rotation elements of the third planetary gear set PG3form a second shift line SP2 such that D2 is output through the thirdplanet carrier PC3.

[Third Forward Speed]

The first brake B1 that was operated at the second forward speed 2ND isreleased and the second clutch C2 is operated at the third forward speed3RD.

As shown in FIG. 3C, the rotation speed of the first shaft IS1 is inputto the first planet carrier PC1, and the first planetary gear set PG1becomes a direct-coupling state by operation of the second clutch C2 andthe fourth clutch C4.

The rotation speed of the first ring gear R1 is reduced according to thegear ratio of the second transfer gear TF2 and is then input to thethird sun gear S3 as an inverse rotation speed by operation of thefourth clutch C4, and the rotation speed of the first sun gear S1 isreduced according to the gear ratio of the third transfer gear TF3 andis then input to the third ring gear R3 as an inverse rotation speed.

Therefore, the rotation elements of the third planetary gear set PG3form a third shift line SP3 such that D3 is output through the thirdplanet carrier PC3.

[Fourth Forward Speed]

The second clutch C2 that was operated at the third forward speed 3RD isreleased and the third clutch C3 is operated at the fourth forward speed4TH.

As shown in FIG. 3D, the rotation speed of the first shaft IS1 is inputto the first planet carrier PC1, the first ring gear R1 is connected tothe second sun gear S2 through the first transfer gear TF1, the secondring gear R2 is connected to the third sun gear S3 through the secondtransfer gear TF2 by operation of the third and fourth clutches C3 andC4, and the first sun gear S1 is connected to the third ring gear R3through the third transfer gear TF3 by operation of the first clutch C1.

Therefore, the rotation elements of the third planetary gear set PG3form a fourth shift line SP4 by operation of the first, second, andthird planetary gear sets PC1, PC2, and PC3 such that D4 is outputthrough the third planet carrier PC3.

[Fifth Forward Speed]

The fourth clutch C4 that was operated at the fourth forward speed 4THis released and the second clutch C2 is operated at the fifth forwardspeed 5TH.

As shown in FIG. 3E, the rotation speed of the first shaft IS1 is inputto the first planet carrier PC1, the first ring gear R1 is connected tothe second sun gear S2 through the first transfer gear TF1, the firstplanet carrier PC1 is connected to the second ring gear R2 and the thirdsun gear S3 through the second transfer gear TF2 by operation of thesecond and third clutches C2 and C3, and the first sun gear S1 isconnected to the third ring gear R3 through the third transfer gear TF3by operation of the first clutch C1.

Therefore, the rotation elements of the third planetary gear set PG3form a fifth shift line SP5 by operation of the first, second, and thirdplanetary gear sets PC1, PC2, and PC3 such that D5 is output through thethird planet carrier PC3.

[Sixth Forward Speed]

The first clutch C1 that was operated at the fifth forward speed 5TH isreleased and the fourth clutch C4 is operated at the sixth forward speed6TH.

As shown in FIG. 3F, the rotation speed of the first shaft IS1 is inputto the first planet carrier PC1, and the first planetary gear set PG1becomes the direct-coupling state by operation of the second clutch C2and the fourth clutch C4.

The rotation speed of the first ring gear R1 is increased according tothe gear ratio of the first transfer gear TF1 and is then input to thesecond sun gear R2 as an inverse rotation speed, and the rotation speedof the first ring gear R1 is reduced according to the gear ratio of thesecond transfer gear TF2 and is then input to the second ring gear R2and the third sun gear S3 as an inverse rotation speed by operation ofthe third and fourth clutches C3 and C4.

Therefore, the rotation elements of the third planetary gear set PG3form a sixth shift line SP6 such that D6 is output through the thirdplanet carrier PC3.

[Seventh Forward Speed]

The fourth clutch C4 that was operated at the sixth forward speed 6TH isreleased and the first brake B1 is operated at the seventh forward speed7TH.

As shown in FIG. 3G, the rotation speed of the first shaft IS1 is inputto the first planet carrier PC1, and the first sun gear S1 is operatedas a fixed element by operation of the first brake B1.

The rotation speed of the first ring gear R1 is increased according tothe gear ratio of the first transfer gear TF1 and is then input to thesecond sun gear S2 as an inverse rotation speed, and the rotation speedof the first planet carrier PC1 is reduced according to the gear ratioof the second transfer gear TF2 and is then input to the second ringgear R2 and the third sun gear S3 as an inverse rotation speed byoperation of the second and third clutches C2 and C3.

Therefore, the rotation elements of the third planetary gear set PG3form a seventh shift line SP7 such that D7 is output through the thirdplanet carrier PC3.

[Eighth Forward Speed]

The second clutch C2 that was operated at the seventh forward speed 7THis released and the fourth clutch C4 is operated at the eighth forwardspeed 8TH.

As shown in FIG. 3H, the rotation speed of the first shaft IS1 is inputto the first planet carrier PC1, and the first sun gear S1 is operatedas a fixed element by operation of the first brake B1.

The rotation speed of the first ring gear R1 is increased according tothe gear ratio of the first transfer gear TF1 and is then input to thesecond sun gear S2 as an inverse rotation speed, and the rotation speedof the first ring gear R1 is reduced according to the gear ratio of thesecond transfer gear TF2 and is then input to the second ring gear R2and the third sun gear S3 as an inverse rotation speed by operation ofthe third and fourth clutches C3 and C4.

Therefore, the rotation elements of the third planetary gear set PG3form an eighth shift line SP8 such that D8 is output through the thirdplanet carrier PC3.

[Reverse Speed]

As shown in FIG. 2, the first brake B1 and the first and third clutchesC1 and C3 are operated at the reverse speed REV.

As shown in FIG. 3I, the rotation speed of the first shaft IS1 is inputto the first planet carrier PC1, and the first sun gear S1 and the thirdring gear R3 are operated as fixed elements by operation of the firstbrake B1 and the first clutch C1.

The rotation speed of the first ring gear R1 is increased according tothe gear ratio of the first transfer gear TF1 and is then input to thesecond sun gear S2 as an inverse rotation speed, and the second ringgear R2 is connected to the third sun gear S3 by operation of the thirdclutch C3.

Therefore, the rotation elements of the third planetary gear set PG3form a reverse shift line RS such that REV is output through the thirdplanet carrier PC3.

Since three planetary gear sets are separately disposed on the firstshaft and the second shaft disposed apart from and in parallel with eachother in the planetary gear train according to an exemplary embodimentof the present invention, a length thereof may be reduced andmountability may be improved.

In addition, optimum gear ratios may be set due to ease of changing gearratios by using three external-meshing gears as well as the planetarygear sets. Since gear ratios can be changed according to targetperformance, starting performance may be improved. Therefore, a start-upclutch instead of a torque converter may be used.

Since three frictional elements are operated at each shift-speed,non-operated frictional element may be minimized and drag torque may bereduced. In addition, fuel consumption may be reduced by increasingpower delivery efficiency.

In addition, since torque load of each frictional element can bereduced, compact design is possible.

For convenience in explanation and accurate definition in the appendedclaims, the terms “upper”, “lower”, “inner” and “outer”, “up,” “down,”“upper”, “lower,” “upwards,” “downwards”, “front”, “rear”, “back”,“inside”, “outside”, “inwardly,” “outwardly,” “interior”, “exterior”,“inner,” “outer”, “forwards” and “backwards” are used to describefeatures of the exemplary embodiments with reference to the positions ofsuch features as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. as well as various alternatives and modifications thereof. Itis intended that the scope of the invention be defined by the Claimsappended hereto and their equivalents.

What is claimed is:
 1. A planetary gear train of an automatictransmission for a vehicle, comprising: a first shaft receiving torqueof an engine; a second shaft disposed in parallel with the first shaft;a first planetary gear set disposed on the first shaft, and including arotation elements thereof a first sun gear selectively operated as anoutput element or a fixed element, a first planet carrier directlyconnected to the first shaft and continuously operated as an inputelement, and a first ring gear operated as an output element; a secondplanetary gear set disposed on the second shaft, and including asrotation elements thereof a second sun gear receiving an inverserotation speed from the first ring gear, a second planet carrierdirectly connected to the second shaft, and a second ring gearselectively connected to the first planet carrier or the first ringgear; a third planetary gear set disposed on the second shaft, andincluding as rotation elements thereof a third sun gear selectivelyconnected to the first planet carrier, the first ring gear, or thesecond ring gear, a third planet carrier directly connected to thesecond shaft and an output gear, a third ring gear selectively connectedto the first sun gear; three transfer gears engaging three rotationelements of the first planetary gear set with selected rotation elementsamong the rotation elements of the second and third planetary gear sets;and five frictional elements selectively interconnecting the rotationelements of the first, second, and third planetary gear sets orselectively connecting the rotation elements to a transmission housing.2. The planetary gear train of claim 1, wherein each of the first,second, and third planetary gear sets is a single pinion planetary gearset.
 3. The planetary gear train of claim 1, wherein the three transfergears comprise: a first transfer gear including a first transfer drivegear connected to the first ring gear and a first transfer driven gearconnected to the second sun gear; a second transfer gear including asecond transfer drive gear selectively connected to the first planetcarrier or the first ring gear and a second transfer driven gearconnected to the third sun gear; and a third transfer gear including athird transfer drive gear selectively connected to the first sun gearand a third transfer driven gear connected to the third ring gear. 4.The planetary gear train of claim 3, wherein the second transfer drivengear is selectively connected to the second ring gear.
 5. The planetarygear train of claim 3, wherein the five frictional elements comprise: afirst brake mounted between the first sun gear and the transmissionhousing; a first clutch mounted between the first sun gear and the thirdtransfer drive gear; a second clutch mounted between the first planetcarrier and the second transfer drive gear; a third clutch mountedbetween the second ring gear and the second transfer driven gear; and afourth clutch mounted between the first ring gear and the secondtransfer drive gear.
 6. The planetary gear train of claim 5, wherein thethird clutch is mounted between the second ring gear and the third sungear.
 7. The planetary gear train of claim 5, wherein the fourth clutchis mounted between the first transfer drive gear and the second transferdrive gear.
 8. The planetary gear train of claim 5, wherein a firstforward speed is achieved by operating the first brake and the first andsecond clutches, a second forward speed is achieved by operating thefirst brake and the first and fourth clutches, a third forward speed isachieved by operating the first, second, and fourth clutches, a fourthforward speed is achieved by operating the first, third, and fourthclutches, a fifth forward speed is achieved by operating the first,second, and third clutches, a sixth forward speed is achieved byoperating the second, third, and fourth clutches, a seventh forwardspeed is achieved by operating the first brake and the second and thirdclutches, an eighth forward speed is achieved by operating the firstbrake and the third and fourth clutches, and a reverse speed is achievedby operating the first brake and the first and third clutches.
 9. Aplanetary gear train of an automatic transmission for a vehicle,comprising: a first shaft receiving torque of an engine; a second shaftmounted in parallel with the first shaft; a first planetary gear setmounted on the first shaft, and including a rotation elements thereof afirst sun gear selectively operated as an output element or a fixedelement, a first planet carrier directly connected to the first shaftand continuously operated as an input element, and a first ring gearoperated as an output element; a second planetary gear set mounted onthe second shaft, and including as rotation elements thereof a secondsun gear receiving an inverse rotation speed from the first ring gear, asecond planet carrier directly connected to the second shaft, and asecond ring gear selectively connected to the first planet carrier orthe first ring gear; a third planetary gear set mounted on the secondshaft, and including as rotation elements thereof a third sun gearselectively connected to the first planet carrier, the first ring gear,or the second ring gear, a third planet carrier directly connected tothe second shaft and an output gear, a third ring gear selectivelyconnected to the first sun gear; a first transfer gear including a firsttransfer drive gear connected to the first ring gear and a firsttransfer driven gear connected to the second sun gear; a second transfergear including a second transfer drive gear selectively connected to thefirst planet carrier or the first ring gear and a second transfer drivengear connected to the third sun gear; a third transfer gear including athird transfer drive gear selectively connected to the first sun gearand a third transfer driven gear connected to the third ring gear; afirst brake mounted between the first sun gear and the transmissionhousing; a first clutch mounted between the first sun gear and the thirdtransfer drive gear; a second clutch mounted between the first planetcarrier and the second transfer drive gear; a third clutch mountedbetween the second ring gear and the second transfer driven gear; and afourth clutch mounted between the first ring gear and the secondtransfer drive gear.
 10. The planetary gear train of claim 9, whereineach of the first, second, and third planetary gear sets is a singlepinion planetary gear set.
 11. The planetary gear train of claim 9,wherein the third clutch is mounted between the second ring gear and thethird sun gear.
 12. The planetary gear train of claim 9, wherein a firstforward speed is achieved by operating the first brake and the first andsecond clutches, a second forward speed is achieved by operating thefirst brake and the first and fourth clutches, a third forward speed isachieved by operating the first, second, and fourth clutches, a fourthforward speed is achieved by operating the first, third, and fourthclutches, a fifth forward speed is achieved by operating the first,second, and third clutches, a sixth forward speed is achieved byoperating the second, third, and fourth clutches, a seventh forwardspeed is achieved by operating the first brake and the second and thirdclutches, an eighth forward speed is achieved by operating the firstbrake and the third and fourth clutches, and a reverse speed is achievedby operating the first brake and the first and third clutches.